Oscar Pistorius, the South African double-amputee Olympian accused of murdering his girlfriend, Reeva Steenkamp, took the stand for the first time Monday, testifying about his faith and his love for the woman he killed.

PRETORIA, South Africa -- “That's the moment that everything changed,” South African Olympian Oscar Pistorius testified in his murder trial Tuesday, describing the instant in the early hours of Valentine’s Day last year when he said he heard the noise of a window opening in his bathroom, a few yards from the bedroom where he had been sleeping with girlfriend Reeva Steenkamp.

He said the only light in his bedroom came from a small LED light on his amplifier, which he had just covered with a pair of Steenkamp’s jeans when he heard the window open.

“I thought there was a burglar that was gaining entry into my home,” he testified in Pretoria’s High Court. The room was pitch black, he said, and he had just gotten out of bed to close a door and reposition two fans on a hot and muggy Pretoria night.

PHOTOS: Oscar Pistorius

“I think initially I just froze. I heard this noise. I interpreted it as someone climbing into the bathroom,” he told the court. He said he was unsure whether there was one intruder or more and imagined them coming the few yards down the passage into his bedroom.

“They could be there at any moment,” he said. “The first thing in my mind was that I needed to arm myself, that I needed to protect Reeva and I, to get my gun,” he said.

He said he grabbed his gun and moved to put himself between the intruder or intruders and Steenkamp.

Pistorius has pleaded not guilty to the murder of Steenkamp, claiming he opened fire on her believing he was shooting an intruder or intruders. He has also pleaded not guilty to two charges of recklessly discharging a gun, and to one charge of unlicensed possession of ammunition.

The double amputee said as he moved toward the passage on his stumps, holding his gun out in front of him, he slowed down, terrified that an intruder might already be in the passage.

He testified that he then ran into the passage: “It was at that point that I was just overcome with fear and I started screaming and shouting for the burglars to get out of my house. I shouted at Reeva to get down on the floor.

“I slowly made my way down the passage, constantly aware that these people or persons could come at me at any time. I didn’t have my legs on,” Pistorius said.

His words tumbled out as he described the his version of events on that night. At times tears rolled down his cheek. At other times Judge Thokozile Masipa had to ask him to slow down.

He then said he stopped shouting in the passage, worried that an intruder would work out where he was and shoot him. Inside the passage, he said, he heard the toilet door slam. He said the noise confirmed to him that there were people or a person in the toilet.

The court adjourned for lunch before Pistorius could describe opening fire and shooting through the door.

It’s known that Pistorius fired four expanding bullets through the door, three of which struck Steenkamp in the hip, arm and head. Experts for the prosecution and the defense have testified that she was hit in the hip first and would have fallen. According to two pathologists who have previously testified in the case, she would have died soon after being shot in the head.

Earlier, Pistorius told the court that he was “besotted” with Steenkamp and was sometimes jealous and insecure about their relationship.

He said that in November 2012 he asked her on a whim to accompany him to a sporting awards function at the last moment. The two stayed up late talking and saw each other every day for six days afterward, he said, but the relationship got serious in January.

“In mid-January everything started to pick up and I’d say that’s when our relationship really got going. We both came out of difficult relationships before. I was very keen on Reeva. I think if anything I was maybe more into her than she was into me. I let her take her space. I was besotted with her,” he said.

Pistorius wept as he read phone messages between him and Steenkamp in court, including a long message from her saying that he occasionally scared her and that she felt the relationship wasn’t equal.

Pistorius said they sometimes quarreled, including an occasion when he was angry because she initially did not introduce him to a male friend she was talking to at a function.